Polyesters have already got widespread recognition as resins for general molding use but because of their inadequacies in various mechanical characteristics, such as flexual strength, flexual modulus, etc., many polyesters are unsuitable for practical applications calling for high performance.
Recent years, there are increasing demands for new materials that have superior in strength, rigidity, heat resistance, resistance to chemicals and so on in fields such as textile fibers, films, general moldings, etc.
As a harbinger of a liquid crystal polyester possessing such high performance characteristics, W. J. Jackson et al. reported on a liquid crystal copolyester consisting exclusively of the ester bonds of ethylene terephthalate and parahydroxybenzoic acid in J. Polm. Sci. Polym. Chem. Ed., 14, 2043 (1976) and U.S. Pat. No. 3,804,805, and there has since been a mounting interest in such liquid crystal polyesters. Compared with ordinary polyesters which does not show liquid crystalinity, the above-mentioned liquid crystal polyester has much superior mechanical characteristics, thus being regarded as a new type of high performance resin. Particularly, those species rich in parahydroxybenzoic acid residues are considered to be promising engineering plastics with extremely high heat resistance.
The method for manufacture of the above polyester is described in U.S. Pat. No. 3,778,410. According to this patent, polyethylene terephthalate and paraacetoxybenzoic acid are first heated in an inert gas atmosphere for acidolysis reaction to form polyester fragments and, then, the pressure of reaction system is reduced to cause a viscosity increase to thereby give the desired product.
However, the manufacture of a liquid crystal polyester by the above method presents the following problems. First, no matter how one attempts to increase the agitation speed or improve the conditions of acidolysis, a highly blocked polymer fraction due to block polymerization of parahydroxybenzoic acid comes out as an insoluble and infusible impurity (hereinafter "homopolymer") whenever the proportion of parahydroxybenzoic acid component is not less than 75 mol%. The result is that the final liquid crystal polyester assumes as if it contained an organic filler and shows a reduced fluidity, thus seriously interfering with the discharging operation after the polymerization reaction. This in turn makes the melt-polymerization substantially impractical and interferes with subsequent processing operations such as injection molding, detracts from the heat resistance and mechanical properties (particularly heat deformation temperature and Izod impact strength) of the final moldings, and leads to a rough molded surface.
As an indicator of the presence of the formation of a homopolymer, the solubility of the product in a 50:50 by weight mixture of phenol and 1,1,2,2-tetrachlorethane (about 60/40 by volume) (hereinafter "solvent mixture") may be utilized. Thus, a homopolymer-rich liquid crystal polyester is either insoluble in the solvent mixture or simply dispersed therein as white turbidity. By way of illustration, according to the above-mentioned report of W. J. Jackson et al., J. Polym. Sci. Polym. Chem. Ed., 14, 2043 (1976), a liquid crystal polyester with a terephthalic acid residue-to-parahydroxybenzoic acid residue mol ratio of 20:80 was insoluble in the solvent mixture and, hence, the intrinsic viscosity determination was not made.
U.S. Pat. No. 3,804,805 claims a polymer with an intrinsic viscosity of not less than 0.4, but the technology described therein is only relevant to polymers soluble in the solvent mixture for viscosity determination and there is no specific statement referring to a liquid crystal polyester containing a high mol proportion of parahydroxybenzoic acid component such as the liquid crystal polyester according to the present invention.
Thus, using known production technology there has never been obtained a liquid crystal polyester which contains the parahydroxybenzoic acid unit in such a high proportion and is yet soluble in solvent mixture substantially without losing its clarity, that is to say, free of the homopolymer arising from the block polymerization of parahydroxybenzoic acid.